While preparing an ophthalmic lens for mounting in an eyeglass frame, it is often necessary to determine the refractive power and the prismatic deflection of the lens.
To do that, it is common practice to use a plate having patterns of the Hartmann plate type that is interposed between the lens and an image capture device. The lens is lighted by lighting means located on a side of the lens opposite from its side where the image capture device is located.
The patterned plate is transparent and includes a matrix of dots in positions that are predetermined in the absence of a lens. By comparing their positions in the presence of the lens and in the absence of the lens it is possible to determine the spherical power of the lens and its prismatic deflection.
Nevertheless, using such a patterned plate presents drawbacks. In particular, inserting the plate between the lens and the image capture device requires the distance between the lens and the image capture device to be increased, thereby degrading the quality of the image and increasing the effects of distortion associated with the power of the lens.
In addition, including such a dedicated patterned plate increases the cost of the device being used.
Furthermore, the dots on the patterned plate are then displayed in the image together with the lens and the test chart. These dots can disturb the user and prevent accurate adjustment, in particular when the measurement is performed simultaneously with other adjustments that require centering or axis-orientation marks of the lens to be identified, since the dots on the plate can overlie the images of these marks.
It is also known to add markers on the lens for determining a refractive characteristic of a lens arranged on a support and imaged on one side of the support under lighting coming from the opposite side of the support.
Adding paint markers on the surface of the lens can also impede reading centering and/or axis-orientation marks on the lens. The quality of the image is thus degraded. In addition, adding such markers is lengthy and difficult for the user.
It is most advantageous to measure the refractive power and the prismatic deflection of a lens when blocking an ophthalmic lens, as explained in greater detail below.
In order to mount an ophthalmic lens in an eyeglass frame, several steps need to be performed:                the shape of the rim of the frame is determined:        the lens is centered and blocked using a centering and blocking device: a gripper peg, also known as a “block”, is fastened on a face of the lens in order to embody a reference position and orientation for trimming the lens and in order to enable the lens to be driven in rotation while it is being trimmed; and        the ophthalmic lens is trimmed with the help of a grinder and it is mounted in the frame.        
The centering and blocking device is adapted to determine a point on the surface of the lens that defines the location where the block is to be placed and/or the orientation that the block is to have as a function of the position of a centering and/or axis-orientation mark on the ophthalmic lens and as a function of client parameters (pupillary distance, height from the bottom of the frame, angle of astigmatism, . . . ).
The centering mark on the lens is at the optical center of the lens for a single vision lens, however it is offset from that optical center for lenses of other types: the centering mark is a centering cross for progressive lenses, or the top of a segment having a different power for bifocal lenses.
By way of example, axis-orientation marks may be constituted by horizontal lines.
Most centering and blocking devices that are already known detect the position of the centering mark and/or of the axis-orientation mark of an ophthalmic lens by lighting the lens with a light beam and by picking up the light beam transmitted through the lens. In the image that is obtained, known devices identify the shadows of the centering and/or axis-orientation marks.
In practice, in such a prior art device, the block is placed on the lens by an articulated arm that always performs the same stroke. The block is thus always positioned in the same position in three dimensions and it is necessary to adjust the position of the lens relative to that position in three dimensions in order to ensure that it coincides with the position desired for the block on the lens.
The image of the lens as captured by the camera situated on one side of the lens while the lens is being lighted by the light source located on the other side of the lens is displayed on a screen. The screen also displays a test chart indicating a position for the centering mark that is determined in such a manner that if the centering mark of the lens is aligned with said test chart, the block will be deposited in a position that is suitable for blocking.
The user then needs only to move the lens in such a manner as to cause the position of the centering mark of the lens to coincide on the screen with the position of the test chart.
Such a method is said to be a “projected view” method since the light rays pass through the lens before being detected by the image capture device.
Such devices lead to errors in detecting the position of the optical center or the positions of the centering and/or axis-orientation marks of the ophthalmic lens. Such errors result from the prismatic deflections of the shadows of the marks as induced by the lens itself, where such deflections depend on the spherical, cylindrical, and prismatic optical powers of the ophthalmic lens in the region of the mark in question.
All light rays passing through the lens are deflected, with the exception of those that pass through the optical center of a single-vision lens. Thus, the image of the ophthalmic lens as displayed on the screen and viewed by the user is deformed from reality, which means that the centering mark of the lens is no longer in the three-dimensional position corresponding to the position displayed on the screen.
By aligning the image of the centering mark with the test chart, a systematic error is introduced into the positioning of the block.
This is also true when determining the orientation of the block relative to the image of the axis-orientation mark of the lens.
One solution to that problem consists in determining one or more refractive characteristics of the lens, such as for example its spherical power, and in correcting the positioning of the block on the lens as a function of that characteristic.
Nevertheless, the use of a pattered plate or of paint markers placed on the lens presents the above-mentioned drawbacks: the dots on the patterned plate run the risk of disturbing the user and of preventing accurate adjustment by being superposed on the images of the centering and axis-orientation marks of the lens.
In addition, the quality of the image is degraded, and adding additional elements in the optical equipment can be expensive and difficult for the user.